Magnetic surveying is well established in land based mineral exploration. Magnetic data is routinely used to map geology in covered terrains, to identify altered zones, mineralization, bedding attitudes, and fault networks. Typically this surveying is done from an airplane, and is used to map geology in covered terrains, to estimate the depth to basement in overburden covered areas, and to identify altered zones, mineralization, bedding attitudes, and fault networks. In ocean based surveys, particularly those under the sea, magnetic surveys are being done in specialized applications. These applications mainly require high resolution magnetic mapping, and Automated Underwater Vehicles (AUVs). AUVs are a good platform for these surveys because they are capable of flying close to the target of interest. In military applications, AUVs are used for naval mine-hunting and unexploded ordinance applications: Sulzberger, Hunting Sea Mines with UUV-Based Magnetic and Electro-Optic Sensors, OCEANS 2009, MTS/IEEE Biloxi, pp. 1-5, and Pei, UXO Survey using Vector Magnetic Gradiometer on Autonomous Underwater Vehicle, OCEANS 2009, MTS/IEEE Biloxi, pp. 1-8, each incorporated by reference in entirety.
Commercial Remotely Operated Vehicles (“ROVs”) and AUV magnetic surveys are also used for undersea pipeline and cable inspections. Research surveys using AUVs in areas of hydrothermal vents and ocean ridges have utilized magnetic data to interpret the nature and geometry of the hydrothermal system beneath the seafloor. See Tivey, The Magnetic Signature of Hydrothermal Systems in Slow Spreading Environments, in Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges, Geophys. Monogr. Ser., vol. 188, edited by P. A. Rona et al., 440 pp., AGU, Washington, D.C., doi:10.1029/2008GM000773, 2010, and Honsho, Deep-sea magnetic vector anomalies over the Hakurei hydrothermal field and the Bayonnaise knoll caldera, Izu-Ogasawara arc, Japan, Journal of Geophysical Research: Solid Earth, v. 118, doi:10.1002/jgrb.50382, 2013, each incorporated by reference in entirety.
However, typically magnetic data is not collected during regular AUV and ROV operations, even when these vehicles are being used for mineral exploration. There are a number of probable reasons.
The teams doing marine mineral exploration are typically hydrographers and geologists, and not normally mineral exploration geophysicists familiar with magnetic data processing and interpretation. As well, magnetometers are sold as a piece of gear FOB factory, so the training necessary to operate the magnetometer, collect, process and interpret the data must be acquired elsewhere. Consequently, a survey including the use of a magnetometer will typically require a specialist added to the survey crew. Additionally, the survey information with the xyz position and attitude of the magnetometer is normally collected separately and needs to be merged with the raw magnetometer data prior to processing.
However, probably the largest reason for not collecting magnetic data during AUV mapping surveys are the magnetic fields produced by the AUV obscure the geological information in maps of the raw magnetic data. The fields produced by a vehicle can be quite large, corrupting and perhaps overwhelming the ambient magnetic field generated by the local geological environment, and are attitude and heading dependent. Proton precession or Overhauser effect magnetometers simply do not work within the high magnetic gradients present within the body of the AUV. Degaussing of the AUV is one way of removing static magnetic vehicle fields but is difficult and will eventually wear off. Another solution to mitigate effects of these fields is to mount the magnetometer away from the AUV with specialized mounting apparatus such as a towed body or long poles. However, this comes with the cost of increased complication to operations and risk to vehicle safety.
The other solution is to mount the magnetometer inside the AUV and compensate for the attitude and heading dependent effects. This requires compensation not only for the attitude of the AUV in the earth's field, but also for secondary effects related to the strength of the electric currents flowing in the vehicle propulsion and control circuits.
However, other than during specialized commercial, military, and academic surveys, magnetic data is not normally collected on autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs) conducting geological mapping and hydrographic survey operations.
One reason for this is the magnetic field produced by the local geology is often overwhelmed by the heading and attitude dependent magnetic fields of the vehicle when the magnetometer is mounted close to or inside the AUV. Magnetometers can be mounted away from the AUV with specialized mounting apparatus e.g. a towed body or pole-mounts, but at the cost of increased complication to operations and risk to vehicle safety. To produce useful data from a magnetometer mounted inside the body of an AUV, it is necessary to compensate not only for the attitude of the AUV in the earth's field, but also for secondary effects related to the strength of the electric currents flowing in the vehicle propulsion and vehicle control circuits.
Traditional devices and methods to address magnetic compensation are limited. For example, U.S. Patent Appl. Publication No. 2008/0125920 to Miles et al., discloses an un-manned airborne vehicle (UAV), for acquiring aeromagnetic data for geophysical surveying at low altitude on land or over water, comprising an extended fuselage that is adapted to hold and maintain magnetometer and a magnetic compensation magnetometer at a minimum distance from the avionics and propulsion systems of the UAV. The magnetometer measures magnetic anomalies and the magnetic compensation magnetometer measures magnetic responses corresponding to the pitch, yaw and roll of the UAV. A data acquisition system stores and removes the magnetic response measurements from the magnetic anomaly measurements. The data acquisition system also stores a survey flight plan and transmits the same to the avionics system. The generator of the UAV is shielded and the propulsion system is stabilized to reduce magnetic and vibrational noises that can interfere with the operation of the magnetometer. Miles does not, for example, disclose a system and method for compensation of magnetic data as collected during autonomous underwater vehicle mapping surveys that does not require the data to be placed in a geographic frame of reference prior to correction terms to be calculated, and which comprises a correction for variable thruster motor currents. Miles is incorporated by reference in its entirety.
U.S. Patent Appl. Publication No. 2014/0152455 to Giori et al., discloses a first object, an autonomous underwater vehicle equipped for the acquisition of the gravimetric and magnetic gradient near the seabed, characterized in that it comprises gravimetric gradiometer and a magnetic gradiometer. In particular, said autonomous equipped underwater vehicle allows underwater explorations as far as 3,000 m. Giori does not, for example, disclose a system and method for compensation of magnetic data as collected during autonomous underwater vehicle mapping surveys that does not require the data to be placed in a geographic frame of reference prior to correction terms to be calculated, and which comprises a correction for variable thruster motor currents. Giori is incorporated by reference in its entirety.
U.S. Pat. No. 8,378,671 to Mahoney discloses cost-effective compact magnetometers which can be deployed across large ocean areas to record magnetic field strengths. Each magnetometer has a canister containing a magnetometer sensor at its upper end to detect magnetic field strengths of magnetic influence sweep systems and provide representative data signals. Each magnetometer also has sensors to collect data representative of the orientation of the magnetometer as well as temperature and depth to aid in post operational evaluation of the gathered magnetic strength data. A computer processor connected to the sensors controls receipt of the data signals and stores them in a memory device. Batteries at the canister's lower end supply power for the sensors, processor, and memory. An anchor release mechanism causes an anchor to separate from the canister, allowing it to float to the surface for recovery or to transmit data via a UHF transceiver. Mahoney does not, for example, disclose a system and method for compensation of magnetic data as collected during autonomous underwater vehicle mapping surveys that does not require the data to be placed in a geographic frame of reference prior to correction terms to be calculated, and which comprises a correction for variable thruster motor currents. Mahoney is incorporated by reference in its entirety.
U.S. Pat. No. 4,995,165 to Daniels discloses a roll-independent magnetometer which may be used in a towed array magnetometer system comprises a first magnetic field sensor having iso-angular flux sensitivity about a roll axis and a second magnetic field sensor having sensitivity along the roll axis, the arrangement being such that a component of a magnetic field along the roll axis is measured by the second unidirectional sensor and the component of the field lateral to the roll axis is measured by the first sensor. Corrections may be applied for the inclination of the sensor and the angle of dip of the Earth's field. The angle of dip may be measured either in the towing vessel, in the towed body, or provided in look-up tabulated form. Daniels does not, for example, disclose a system and method for compensation of magnetic data as collected during autonomous underwater vehicle mapping surveys that does not require the data to be placed in a geographic frame of reference prior to correction terms to be calculated, and which comprises a correction for variable thruster motor currents. Daniels is incorporated by reference in its entirety.
U.S. Pat. No. 4,109,199 to Ball discloses a three axis magnetometer with a single calibration checking coil lying in a plane disposed at equal angles to each of the three orthogonal axes of sensitivity. Energization of the calibration checking coil with a known current while the calibrated magnetometer is in a known condition of calibration provides sensitivity readings for each of the three axes, which readings provide a standard of comparison for checking the calibration and sensitivity of the magnetometer by similar energization of the coil when the magnetometer is disposed in a remote operational environment. Ball does not, for example, disclose a system and method for compensation of magnetic data as collected during autonomous underwater vehicle mapping surveys that does not require the data to be placed in a geographic frame of reference prior to correction terms to be calculated, and which comprises a correction for variable thruster motor currents. Ball is incorporated by reference in its entirety.
By way of providing additional background, context, and to further satisfy the written description requirements of 35 U.S.C. § 112, the following references are incorporated by reference in their entireties: U.S. Patent Appl. Publication No. 2014/0165898 to Cierpka; WO 2012/068362 to Sheng; U.S. Patent Appl. Publication No. 2011/0010095 to Dyer; U.S. Patent Appl. Publication No. 2013/0239869 to Hesse; U.S. Pat. No. 6,765,383 to Barringer; Wo 1999/050619 to Ceccherini; and U.S. Pat. No. 8,148,992 to Kowalczyk.
Thus, there is a long-felt need for a system and method compensating the magnetic data for the vehicle related fields. This method includes both a physical calibration procedure and a mathematical treatment of the data. This calibration procedure may be performed prior to every survey, and therefore, the addition or subtraction of equipment between launches will not affect final results. Experimental results are disclosed derived from a case study in an area of seafloor hydrothermal venting that shows applying these correction terms to the raw magnetic data produces very useful magnetic maps for the subsurface geology in a survey area.